Why was JJ Thompson important?
JJ Thomson discovered electrons in atoms. This discovery indicated that atoms were not indivisible, which brought about a change in the atomic theory.
Look the atom was first drawn by John Dalton which was like a shell. JJ Thompson discovered it by conducting Thomas Alva Edison's Cathode Ray experiment. He suggested that the atom was covered with electrons like pimples on someones face. Since John Dalton suggested that the atom was electrically neutral JJ Thompson proposed that The atom was positively charged with the negatively charged electron on the body of the atom. Like Pimples.
Well, J.J. Thompson was the first to discover the existence of a negatively charged particle, which turned out to be the Electron. So, to answer your question, it was J.J. Thompson. Also note that JJ thought that de electrons where INSIDE the atom and envelloped in a positively charged layer. P.S. It was 1st called a corpuscle by Thompson and was later changed by another scientist to electron.